Glycerophospholipids, the main component of biological membranes, contain a glycerol core with fatty acids attached as R groups at the sn-1 position and sn-2 position, and a polar head group joined at the sn-3 position via a phosphodiester bond. The specific polar head group determines the name given to a particular glycerophospholipid (e.g., a chloline head group results in a phosphatidylcholine). Glycerophospholipids possess tremendous diversity, not only resulting from variable phosphoryl head groups, but also as a result of differing chain lengths and degrees of saturation of their fatty acids. Generally, saturated and monounsaturated fatty acids are esterified at the sn-1 position, while polyunsaturated fatty acids are esterified at the sn-2 position.
Glycerophospholipid biosynthesis, summarized in U.S. Pat. Appl. Publ. No. 2010-0317882-A1, requires a variety of acyltransferases, including glycerol-3-phosphate acyltransferase (GPAT) [E.C. 2.3.1.15], acyl-CoA:lysophosphatidic acid acyltransferase (LPAAT) [E.C. 2.3.1.51], diacylglycerol acyltransferase (DGAT) [E.C. 2.3.1.20] and phospholipid:diacylglycerol acyltransferase (PDAT) [E.C.2.3.1.158].
Following their de novo synthesis, glycerophospholipids can undergo rapid turnover of the fatty acyl composition at the sn-2 position. This “remodeling”, or “acyl editing”, is important for membrane structure and function, biological response to stress conditions, and manipulation of fatty acid composition and quantity in biotechnological applications. Specifically, the remodeling has been attributed to a combination of deacylation and reacylation of glycerophospholipid. For example, in the Lands' cycle (Lands, J. Biol. Chem., 231:883-888 (1958)), remodeling occurs through the concerted action of: 1) a phospholipase, such as phospholipase A2, that releases fatty acids from the sn-2 position of phosphatidylcholine; and 2) acyl-CoA:lysophospholipid acyltransferases [“LPLATs”], such as acyl-CoA:lysophosphatidylcholine acyltransferase [“LPCAT”] that reacylates the lysophosphatidylcholine [“LPC”] at the sn-2 position (thereby removing acyl-CoA fatty acids from the cellular acyl-CoA pool and acylating lysophospholipid substrates at the sn-2 position in the phospholipid pool). Remodeling has also been attributed to reversible LPCAT activity (Stymne and Stobart (Biochem J., 223(2):305-314 (1984))
The effect of LPCATs (and other LPLATs that have LPCAT activity) on polyunsaturated fatty acid [“PUFA”] production has been contemplated, since fatty acid biosynthesis requires rapid exchange of acyl groups between the acyl-CoA pool and the phospholipid pool. Specifically, desaturations occur mainly at the sn-2 position of phospholipids, while elongation occurs in the acyl-CoA pool. More specifically, U.S. Pat. No. 7,932,077 hypothesized that acyltransferases, including PDAT and LPCAT, could be important in the accumulation of PUFAs (e.g., eicosapentaenoic acid [“EPA”], 20:5 omega-3) in the TAG fraction of Yarrowia lipolytica. As described therein, this was based on the following studies: 1) Stymne and Stobart (Biochem J., 223(2):305-314 (1984)), who hypothesized that the exchange between the acyl-CoA pool and PC pool may be attributed to the forward and backward reaction of LPCAT; 2) Domergue et al. (J. Biol. Chem., 278:35115-35126 (2003)), who suggested that accumulation of gamma-linolenic acid [“GLA”] at the sn-2 position of phosphatidylcholine [“PC”] and the inability to efficiently synthesize arachidonic acid [“ARA”] (20:4 omega-6) in yeast was a result of the elongation step involved in PUFA biosynthesis occurring within the acyl-CoA pool, while delta-5 and delta-6 desaturation steps occurred predominantly at the sn-2 position of PC; 3) Abbadi et al. (The Plant Cell, 16:2734-2748 (2004)), who suggested that LPCAT plays a critical role in the successful reconstitution of a delta-6 desaturase/delta-6 elongase pathway, based on analysis of the constraints of PUFA accumulation in transgenic oilseed plants; and 4) Intl. Appl. Publ. No. WO 2004/076617 A2 (Renz et al.), who provided a gene encoding LPCAT from Caenorhabditis elegans (T06E8.1) that substantially improved the efficiency of elongation in a genetically introduced delta-6 desaturase/delta-6 elongase pathway in S. cerevisiae fed with exogenous fatty acid substrates suitable for delta-6 elongation. Renz et al. concluded that LPCAT allowed efficient and continuous exchange of the newly synthesized fatty acids between phospholipids and the acyl-CoA pool, since desaturases catalyze the introduction of double bonds in PC-coupled fatty acids while elongases exclusively catalyze the elongation of CoA-esterified fatty acids (acyl-CoA).
U.S. Pat. Appl. Publ. No. 2010-0317882-A1 provided further support that LPCAT is indeed important in the accumulation of EPA and docosahexaenoic acid [“DHA”] (22:6 omega-3) in the TAG fraction of Yarrowia lipolytica. It was found that over-expression of LPCATs can result in an improvement in the delta-9 elongase conversion efficiency and/or delta-4 desaturase conversion efficiency (wherein conversion efficiency is a term that refers to the efficiency by which a particular enzyme can convert substrate to product). Thus, in a strain engineered to produce EPA, improvement in delta-9 elongase conversion efficiency was demonstrated to result in increased EPA % TFAs or EPA % DCW. Similarly, improvement in delta-9 elongase and/or delta-4 desaturase conversion efficiency in a strain engineered to produce DHA was demonstrated to result in increased DHA % TFAs or DHA % DCW.
Numerous other references generally describe benefits of co-expressing LPLATs with PUFA biosynthetic genes to increase the amount of a desired fatty acid in the oil of a transgenic organism, increase total oil content, or selectively increase the content of desired fatty acids (e.g., Intl. Appl. Publication Nos. WO 2004/087902, WO 2006/069936, WO 2006/052870, WO 2009/001315, WO 2009/014140). However, none of these references describe the benefits achieved in an organism engineered for high-level production of LC-PUFAs when an LPCAT and a phospholipid:diacylglycerol acyltransferase (PDAT) are both over-expressed. PDAT is an enzyme responsible for transferring a fatty acyl-group from the sn-2 position of a phospholipid (e.g., phosphatidylcholine) to the sn-3 position of 1,2-diacylglycerol to produce a lysophospholipid and TAG via an acyl-CoA-independent mechanism.
Furthermore, despite reports of a variety of conserved membrane bound O-acyltransferase [“MBOAT”] family protein motif sequences within LPCATs in both public and patent literature, a detailed investigation concerning specific mutations within these motifs has not been previously conducted.